For reasons of safety and of environmental protection, especially with the arrival of novel biofuels, motor vehicle constructors are imposing on the pipes mentioned previously particular mechanical characteristics, and also characteristics of very low permeability and of good resistance to the various constituents of fuels, which vary from country to country (hydrocarbons, additives, alcohols such as methanol and ethanol, the alcohols possibly being predominant components in certain cases), the engine lubrication oils and the other chemical products that may be encountered in this environment (battery acid, brake liquids, cooling liquids, metal salts such as calcium chloride or zinc chloride).
The characteristics of the specifications commonly required by motor vehicle constructors for a pipe to be considered satisfactory are cumulatively the following:                good and long-lasting adhesion between the layers, if the pipe is a multilayer pipe, most particularly after having been exposed to fuel;        good integrity of the connections (pipes+joints) after circulation of fuel, i.e. not leading to any leaks;        good dimensional stability of the pipe, when it is used with gasoline;        good resistance to cold shocks (from −30° C. to −40° C. approximately), so that the pipe does not break;        good heat resistance (approximately 150° C.), so that the pipe does not become deformed;        good resistance to aging in a hot oxidative medium (for example: hot air of the engine compartment, from 100 to 150° C. approximately);        good resistance to fuels and to their degradation products and especially with high contents of peroxide;        very low permeability to fuels, and more particularly good biofuel barrier properties, as regards both its polar components (such as ethanol) and its apolar components (hydrocarbons);        good flexibility of the pipe to facilitate mounting, especially of the fuel feed pipework;        
good resistance to ZnCl2 (for example in winter, when roads are gritted, the exterior of the pipe being exposed to this environment).
Furthermore, the desired pipes must avoid the following drawbacks:                if the pipe is a multilayer pipe, peeling of the layers, especially the inner layers, especially during the insertion of a joint (which may lead to leaks);        excessive swelling of the pipe after aging in gasoline/diesel systems (including for biodiesels or biofuels), which may lead to leaks or problems of positioning under the vehicle.        
At the present time, two types of pipe exist, monolayer and multilayer pipes, i.e., pipes consisting of one or more layers of polymer.
Conventionally, the pipes used are manufactured by mono-extrusion, which is the case for a monolayer pipe, or by coextrusion of the various layers, which is the case for a multilayer pipe, according to the usual techniques for transforming thermoplastics.
To ensure good dimensional stability of a multilayer pipe, it is essential to have excellent adhesion between the various polymer layers forming the tube. Most conventionally, an adhesive layer is interposed between two polymer layers, which, by virtue of their composition, do not or do not sufficiently adhere together, to satisfy the specifications mentioned previously.
More generally, the problem to be solved is that of combining highl-carbon polyamide materials, which are materials that are very flexible and very tough (in particular with regard to cold shock, aging in hot air, resistance to zinc chloride), which will generally constitute the outer part of the pipe, with barrier materials, i.e. materials that are sparingly permeable to liquids, which will constitute the inner face of the tube and occasionally come into direct contact with the liquids, such as gasoline or other fluids mentioned previously.
These barrier materials may be weakly carbonic polyamides, which are preferably semicrystalline and with a high melting point, but also non-polyamide barrier materials such as the copolymer of ethylene and vinyl alcohol (denoted EVOH below), or even functionalized fluoro materials such as functionalized polyvinylidene fluoride (PVDF), the functionalized copolymer of ethylene and tetrafluoroethylene (ETFE), the functionalized copolymer of ethylene, tetrafluoroethylene and hexafluoropropylene (EFEP), functionalized polyphenylene sulfide (PPS), or functionalized polybutylene naphthalate (PBN).